aneri says:
<div class="flip ">
                <div class="front">
                    <div id="Slider3" class="multiple-items" runat="server">
                    </div>
                </div>
 replace above code with below
<div id="Slider3" class="multiple-items" runat="server">
</div>
 
aneri says:
 string fileName = Path.GetFileName(filePath);
            strHtm += @"<div id='DivNew" + i + @"'>
                            <img src='images/pic/" + fileName + @"'  />
                        </div>";
replace above code with below
string fileName = Path.GetFileName(filePath);
strHtm += @"<div style='width: 200px; height: 200px' id='DivNew" + i + @"'>
            <div class='flip'>
            <div class='front'>
            <img id='image' src='images/pic/" + fileName + @"'  />
            </div>
            <div class='back'>
            <h3> Some Text </h3></div>
            </div></div>";